Over the past few decades, our knowledge of jets produced by active galactic nuclei (AGN) has greatly progressed thanks to the development of very-long-baseline interferometry (VLBI). Nevertheless, the crucial mechanisms involved in the formation of the plasma flow, as well as those driving its exceptional radiative output up to TeV energies, remain to be clarified. Most likely, these physical processes take place at short separations from the supermassive black hole, on scales which are inaccessible to VLBI observations at centimeter wavelengths. Due to their high synchrotron opacity, the dense and highly magnetized regions in the vicinity of the central engine can only be penetrated when observing at shorter wavelengths, in the millimeter and sub-millimeter regimes. While this was recognized already in the early days of VLBI, it was not until the very recent years that sensitive VLBI imaging at high frequencies has become possible. Ongoing technical development and wide band observing now provide adequate imaging fidelity to carry out more detailed analyses. In this article, we overview some open questions concerning the physics of AGN jets, and we discuss the impact of mm-VLBI studies. Among the rich set of results produced so far in this frequency regime, we particularly focus on studies performed at 43 GHz (7 mm) and at 86 GHz (3 mm). Some of the first findings at 230 GHz (1 mm) obtained with the Event Horizon Telescope are also presented.

在过去的几十年里，由于超长基线干涉测量（VLBI）的发展，我们对活动星系核（AGN）产生的喷流的了解有了很大的进步。然而，等离子体流形成以及驱动其异常辐射输出高达 TeV 能量的关键机制仍有待澄清。最有可能的是，这些物理过程发生在距超大质量黑洞很短的距离处，其尺度是厘米波长下的 VLBI 观测无法达到的。由于其高同步加速器不透明度，中央发动机附近的密集且高度磁化的区域只有在毫米和亚毫米范围内的较短波长下进行观察时才能被穿透。虽然这一点在 VLBI 的早期就已经被认识到，但直到最近几年，高频的灵敏 VLBI 成像才成为可能。持续的技术发展和宽带观测现在提供了足够的成像保真度来进行更详细的分析。在本文中，我们概述了有关 AGN 喷流物理学的一些悬而未决的问题，并讨论了毫米-VLBI 研究的影响。迄今为止在此频率范围内产生的丰富结果中，我们特别关注在 43 GHz (7 mm) 和 86 GHz (3 mm) 下进行的研究。还介绍了事件视界望远镜在 230 GHz（1 毫米）下获得的一些初步发现。